Magnetic Field - Any Insights (Please)

[Benny Sampson]

Okay, here it goes...... I don't know how scientific this experiment was but I did want to duly report back my observations. I purchased an orienteering compass and basically set it on the ground. What I was able to assess is that at a 5 foot distance (having used 3 different sets of speakers) I was not able to affect the needle of the compass; However, at 4 feet I was able to influence the movement of the needle moderately by simply rotating the speaker cabinet; Now at 3 feet I was able to more readily control the needle's movement by rotating either of the cabinets which I used for testing purposes. So, I need to decipher what all this means. How much damage have I done to my beloved reel to reel tape collection? Have only the "highs" been compromised? Damn!!!!! I going to go and test a few tapes by listening and report back.

[Chris Davis]

Originally Posted by Dean Sensui

If you have a dime that gets attracted by a magnet, then you have a very rare collector's item.

dimes are made of non-ferrous metals. If you have one that's made of a ferrous metal then you either have a dime that's one in a billion or a counterfeit coin! :-)

Actually, our poster is from Quebec and Canadian coins are attracted by a magnet. Since 2000, the Canadian dime has been made from 92% steel, and is therefore quite magnetic.

[Benny Sampson]

Thanx for the information Chris, I did not know that :) I took a random sampling of some of the reel to reel music tapes and they seemed to play fine. I have no idea what's going on; My compass experiment demonstrated that as close as four feet the speaker cabinet could effectively influence the needle. Question is, is the force being emitted at 4, 3, & 2 feet from the magnet in the speaker cabinet enough to adversely affect the recordings?

[Allan Black]

Benny, any compass is extremely sensitive and yours could be reacting to the screws or metal frame of your speaker cabinet. Playing your tapes was the right test.

Cheers.

[Benny Sampson]

Thanx for having taken the time to reply Allan :) I do not know how much merit the validity of this test was since you are indeed correct that it could have been any number of elements in the cabinet that would have influenced the needle to fluctuate. However, I also cannot be certain as to what degree my tapes were adversely affected (if any.....) because they did play; This is exactly why I had inquired as to whether anyone knew of an economical device that could help me to render a concrete reading of how strong a magnetic field was being emitted from the speakers. I am uncertain as to what to do next.

[Allan Black]

Ok try this. On a piece of new tape at the end of a reel, record a minute of music with high frequencies in it, at 7.5 IPS.

Play it back and memorise what it sounds like, then cut it out of the reel and wind it onto a small reel. Now move that in very close to the back of your speaker cabinet, wave it around and play it again. If there's no difference to the sound, take the back off your speaker box and wave that tape around a foot from the rear of your woofer coil, Play it.. no difference? wave it *right up against/across/around that speaker coil for 10secs* and play it.

Now I think you might hear something, a lack of highs or just maybe, nothing at all.

[Marco Leavitt]

Dang Benny, you're clearly concerned about this. I think it's time to move either the tapes or the speakers. It doesn't sound like this is something you want to take a chance with.

[Benny Sampson]

Hmmmmmmm, now I don't know what to think. This procedure took awhile because I wanted to do it right. I decided to take heed to Allan's suggestion and duly recorded a 60 second passage at 7.5 i.p.s. with several high frequency sequences. I repeatly played it until I had a firm grasp of the actual sound. My first test entailed moving the reel around directly on the rear wood panel of the speaker cabinet for appoximately 30 seconds - I played back the tape and noted no change in sound. Next, I removed the rear wood panel of the cabinet and waved the reel 5 to 6 inches away from the magnet of the woofer for approximately 30 seconds - I played it back and was not able to hear any difference. The next incremental step was to place the reel directly on the surface of the magnet (3 inches in diameter) and moving it around for approximately 30 seconds - I played it back several times with scrutiny and was not able to hear any difference. I was completely disarmed with the results. Almost wanting to dispel these unanticipated results I proceeded to remove the rear panel of my largest speaker cabinet; The woofer's magnet is 5 inches in diameter and almost 1 inch thick. I placed the same reel directly on this large magnet and moved it around for over 1 minute - I played it back and much to my surprise, I was not able to hear any discernable difference in the sound. What is going on? I was absolutely certain that "something" was going to happen. What is going on? I have no reason to doubt Ben, for instance, when he states than during his radio days they were taught that a sure way to ruin a days work was to store a tape near or on a speaker cabinet. I am very much looking forward to your opinions. Thanx

[Allan Black]

Benny, hope that answers your question, shows your tapes are safe stored where they are and hopefully puts your mind at rest, although I did think you may hear a very slight loss of high frequencies with the last test. So it shows what a low strength magnetic field your biggest speaker puts out.

To erase recorded audio tapes the magnetic field strength of your speaker would have to be 2 to 3 times higher than the coercivity (magnetic field strength Oe) of your tapes. The average audiotape has a rating of 330 Oe and consumer videotape 650 Oe, in line with that videotape needing a bigger degaussing current to erase it.

So you'd need something with a rating of 660-1000 Oe to erase your tapes and 1300-1950 Oe to erase consumer video tape. And that's running the tape over the degaussing rig with no masking medium like a tape box. That's one big degausser.

I've still got one of the small AC powered degaussers we used in our studios. When you put a large pair of metal scissors on it and turn it on, they clamp down so tight to it I've got to prize them off with both hands. 8 inches away there's no magnetic attraction at all. Yet a full 7" reel of 1/4" analogue audio tape needs a slow revolve on the platen to make sure it's fully erased.

When we were training newbies at 2UE, we'd use warnings like .. 'leave your wrist watch out here before you go in there, the degausser could drag the hands off it' Frighten the bejeezes outa them to hopefully save a valuable master tape in the future.

Cheers.

[Benny Sampson]

Hello again. And, Thanx Very Much Allan. This experiment was indeed enligtening. What really intriques me is why all of the dire warnings that are floating around with regards to just how sensitive these tapes really are when in reality it takes quite a significant bit of juice to get them compromised. Is most of the information that's exchanged on his matter moreover based on urban legends?

[Allan Black]

Originally Posted by Benny Sampson

Is most of the information that's exchanged on his matter moreover based on urban legends?

Others might disagree but basically yes! It builds up over the years like a lot of other things. Some businesses make their livlehood with it, drinking bottled water is one. We all grew up with domestic tap water so what's that about lol.

When you put your tape recorder into rec. mode you can imagine the current the erase head puts out to erase the tape passing over it. But that head is only a few inches from the record head itself and it's not affected by the erase head current.

O/T But magnetic underlay sleep mattresses work, get a good brand, it takes about 6months but they do work. :)

Cheers.

[Benny Sampson]

How interesting. Thanx Very Much for all of your insights Allan, they are indeed all greatly appreciated. I was under the distinct impression, as it appears that other members here also were, that this issue of proper distance to a tape recording medium was of imperaive importance. Can anyone here give me an estimated idea as to just how much coercivity an "average" speaker magenet has in terms of Oe force?

[Benny Sampson]

Greetings to All :) Surely some enlightened soul here on the forum can entertain my last query; I have had the privilege as a new member to come across many genuinely kind and insightful people. Thanx

[Andrew Smith]

Hi Alan,

Speaking of others in a radio studio, I used to put the bulk eraser to the side of my head (in line with the eyes) and press the button. No matter how safe I proclaimed it was, I could never get any of the others to do it as well.

There was this issue of one's vision wibbeling while the bulk eraser was on, but that's about it. :-)

Andrew

[Benny Sampson]

Ha! Degaussing body parts too :)